A semiconductor device may be fabricated on a semiconductor wafer having a semiconductor substrate using a variety of processing steps. Etching is a commonly-used process. Etching involves removal of material from the semiconductor substrate and/or selected thin film layers which were previously deposited on the surface of the semiconductor substrate. For example, a chemical in a plasma state may be used to etch selected portions of a nitride layer deposited on a semiconductor wafer.
The rate at which a layer of material, such as a nitride layer, is removed depends on a combination of process parameters such as, for example, RF power, gas flow rates and pressure within a reaction chamber. Each of these process parameters may vary slightly on a run-to-run basis. This may cause unacceptable changes in the etch rate. If the etch rate increases, the etch process may remove too much material. Alternatively, if the etch rate decreases, the etch process may not remove enough material.
The rate at which a layer of material is removed from a semiconductor wafer may not be uniform over the entire surface area of the layer. Consequently, etching a nitride layer with a non-uniform etch rate may cause one of two potential problems. First, the etch process may remove more than just the nitride layer at various points on the wafer. Alternatively, portions of the thin nitride layer may remain after the etch process is complete.
Existing systems and methods for run-to-run control of semiconductor processing are time consuming. Using existing systems and methods, a wafer must be completely processed, measured and the results evaluated before the process parameters may be adjusted to correct the etch process for the next run. Existing systems and methods use ex-situ sensors to measure quality characteristics to provide run-to-run control of the etch process.